1. Field of the Invention
The present invention relates to a rotation angle detector mainly used for detecting a rotation angle of a steering wheel of an automobile.
2. Background Art
Automobiles are becoming increasingly functional in recent years. As a part of such a move, more and more automobiles are using a rotation angle detector to detect the rotation angle of the steering wheel for break control, skid control, and other purposes. A conventional rotation angle detector is described as follows with reference to FIGS. 6 and 7. FIGS. 6 and 7 are a perspective view and a block circuit diagram, respectively, of the conventional detector.
In the conventional rotation angle detector, rotator 31 is provided with spur gear 31A on its outer periphery and engaging portion 31B on its inner periphery. Engaging portion 31B is engaged with a steering wheel shaft (unillustrated) that is inserted into rotator 31. First detecting rotator 32 is provided on its outer periphery with spur gear 32A, and second detecting rotator 33 is provided on its outer periphery with spur gear 33A having a different number of teeth from spur gear 32A. Rotator 31 and first detecting rotator 32 are engaged with each other, and first and second detecting rotators 32 and 33 are engaged with each other. First and second detecting rotators 32 and 33 have magnet 34 and magnet 35, respectively, in their centers which are attached by insert molding or the like. Wiring board 36 is placed nearly parallel to the top surfaces of first and second detecting rotators 32 and 33, and is provided with a plurality of wiring patterns (unillustrated) on its top and bottom surfaces. Wiring board 36 is further provided with anisotropic magnetoresistive element (hereinafter, AMR element) 37 and AMR element 38 on its surface facing magnet 34 and magnet 35, respectively.
As shown in FIG. 7, AMR element 37 includes first Wheatstone bridge 41 and second Wheatstone bridge 42 each of which consists of four magnetoresistive elements 39 connected in a substantially rectangular shape. First and second Wheatstone bridges 41 and 42 are laid upon each other at an inclination of 45 degrees. Power supply terminals 41A and 42A are each derived from one of the four connection points of magnetoresistive elements 39 in each Wheatstone bridge and led to a power supply of 5V. Ground terminals 41B and 42B are each derived from the connection point that is diagonal to the aforementioned connection point and led to the ground.
Each of negative output terminals 41C and 42C and each of positive output terminals 41D and 42D are derived from the two other diagonal connection points of the four connection points and led to amplifier 43 on wiring board 36. Amplifier 43 has output terminals 43A and 43B connected to controller 44 formed of a microcomputer or the like. Note that AMR element 38 has the same structure as AMR element 37. In this manner, the rotation angle detector is structured.
Controller 44 in this type of rotation angle detector is connected to an electronic circuit (unillustrated) housed within the automobile main body through a connector (unillustrated) or the like.
In the aforementioned structure, when a steering wheel (unillustrated) is rotated with the ignition switch (unillustrated) on, rotator 31 starts rotation and causes first and second detecting rotators 32 and 33 to rotate in synchronization therewith.
The rotations of first and second detecting rotators 32 and 33 change the magnetic directions of magnets 34 and 35 attached in their centers. AMR elements 37 and 38 detect the changes. In the case of, for example, AMR element 37, negative output terminal 42C outputs a sine-wave signal to amplifier 43, and positive output terminal 42D outputs a sine-wave signal that is 90 degrees out of phase with the sine-wave signal of negative output terminal 42C to amplifier 43. On the other hand, negative output terminal 41C outputs a cosine-wave signal to amplifier 43, and positive output terminal 41D outputs a cosine-wave signal that is 90 degrees out of phase with the cosine-wave signal of negative output terminal 41C to amplifier 43. In other words, second Wheatstone bridge 42 of AMR element 37 outputs the sine-wave signals, and first Wheatstone bridge 41 at an inclination of 45 degrees from second Wheatstone bridge 42 outputs the cosine-wave signals in accordance with the magnetic direction of magnet 34 that changes with the rotation of first detecting rotator 32.
AMR element 38 outputs sine-wave signals and cosine-wave signals to amplifier 43 in the same manner as in the case of AMR element 37. However, second detecting rotator 33 is different from first detecting rotator 32 in the number of teeth. Therefore, the sine-wave and cosine-wave signals that are outputted to amplifier 43 have a phase different from the outputs of AMR element 37. Amplifier 43 differentially amplifies these sine-wave and cosine-wave signals and then outputs the amplified signals to controller 44.
Controller 44 calculates the signals and detects the rotation angle of rotator 31 from the rotation angles of first and second detecting rotators 32 and 33. As a result, controller 44 detects the rotation angle of the steering wheel. A rotation angle detector of this type is disclosed in Japanese Patent Unexamined Publication No. 2006-29792.
In the above-described rotation angle detector, however, when a conductive foreign matter such as metal powder enters and is present near AMR elements 37 and 38, an output terminal of amplifier 43 may output a signal that is not a sine wave or cosine wave. This can happen, for example, when a short circuit occurs between adjacent output terminals, such as between negative and positive output terminals 41C and 41D or between negative and positive output terminals 42C and 42D. Controller 44, however, cannot recognize this as an abnormal output signal.
When a short circuit occurs between such adjacent output terminals, the output signal has a waveform with a constant voltage. However, at a certain angle, the sine waveforms and the cosine waveforms have the same voltage as the constant voltage. Therefore, controller 44 cannot distinguish the sine-wave and cosine-wave output signals from the short circuit output signal, thereby causing erroneous detection of the rotation angle.